Automatic transmissions of the type addressed by this invention, include gear elements for defining several different forward speed ratios between input and output shafts of the transmission, and electrohydraulic controls for shifting among the various ratios in relation to vehicle speed and load indications. The shifting is effected with a number of fluid operated torque transmitting devices, referred to herein as clutches, which the controls engage and disengage according to a predefined pattern to establish a desired speed ratio.
The various speed ratios of the transmission are typically defined in terms of the ratio Ni/No, where Ni is the input shaft speed and No is the output shaft speed. Speed ratios having a relatively high numerical value provide a relatively low output speed and are generally referred to as lower speed ratios; speed ratios having a relatively low numerical value provide a relatively high output speed and are generally referred to as upper speed ratios. Accordingly, shifts from a given speed ratio to a lower speed ratio are referred to as downshifts, while shifts from a given speed ratio to a higher speed ratio are referred to as upshifts.
A first aspect of shift control is shift scheduling, also known as shift pattern generation. This function is generally carried out by comparing specified vehicle operating parameters (speed and load) to predefined thresholds to determine when shifting is appropriate. Multiple sets of predefined thresholds may be used in connection with a driver preference (Normal/Performance) switch, or control logic which infers the driving style of the operator.
A second aspect of shifting is fluid pressure control. In most transmissions having electrohydraulic controls, the fluid pressure output of a driven pump is regulated to a scheduled pressure (line pressure) and then distributed to the various clutches of the transmission via electrically operated shift valves and timing devices such as hydraulic accumulators. The scheduled pressure is generally speed and load (torque) dependent, and operates not only to maintain adequate torque capacity in engaged clutches, but to control clutch engagement rate during shifting. Since the clutch engagement rate affects shift feel, certain transmission controls increase the normally scheduled pressure, at least during shifting, when a sporty or performance shift feel is desired.
In most transmission pressure controls, an adaptive trim or correction of the scheduled pressure can be employed as a means of compensating for variability associated with part-to-part tolerances, wear, etc. One such control, set forth in the U.S. Pat. No. 4,283,970 to Vukovich, issued Aug. 18, 1981, and assigned to the assignee of the present invention, develops an adaptive correction of the scheduled line pressure based on a deviation of the actual shift time from a desired shift time characteristic of high quality shift feel. In such a system, alternate desired shift time schedules may also have to be employed, depending on whether Normal or Performance pressures are selected.